1. Field of the Invention
The present invention relates to a cooling system for electrical devices; more particularly, to a liquid cooled integrated circuit assembly and a cooling system for a plurality of integrated circuit assemblies.
2. Description of the Related Art
High-density multi-layer printed wiring boards suitable for surface mounting of electrical devices have been developed to provide increasingly smaller spacings between devices and increasingly larger pin densities and pin counts per device. The continuing trend towards greater integration of electrical devices on one interconnection mechanism, for example, a printed circuit board, and the continuing need for increased pulse propagation velocity has caused many features of conventional multi-layer printed wiring boards to approach physical limits. One limiting factor is the transferring away of the heat generated by electrical devices, e.g., integrated circuit packages mounted on the printed circuit board.
As used herein, the following terms have the following meanings: "integrated circuit" means a die; "integrated circuit package" means a structure including an integrated circuit, a package for encapsulating the integrated circuit and having a heat sink; "heat sink" means any portion of a package designed to transfer heat away from an integrated circuit to a coolant, e.g., air or liquid; and "integrated circuit assembly" means a plurality of integrated circuit packages mounted on a supporting structure and a coolant supply system for providing coolant to the integrated circuit packages.
In a conventional integrated circuit assembly, as shown in FIG. 1, a heat transfer element 14 is placed in contact with an integrated circuit package 12 by pressure from a spring or a bellows 16. Heat generated by the operation of the integrated circuit (not shown) enclosed in integrated circuit package 12 is transferred from the integrated circuit to the heat sink portion, from the heat sink to the heat transfer element 14, and from the heat transfer element 14 to a coolant (usually a liquid or gaseous coolant). Conventional cooling systems have problems which include: poor surface contact between the integrated circuit package 12 and the heat transfer element 14, resulting in a large and non-uniform thermal contact resistance; and changes in the thermal contact resistance between integrated circuit package 12 and heat transfer element 14, and thus unstable heat dissipation, due to changes in the pressure applied by the spring or bellows 16. These problems result in relatively poor heat transfer efficiency.
A cooling system for integrated circuit packages which is intended to overcome some of these problems is disclosed in U.S. Pat. No. 4,729,060, Yamamoto, et. at. FIG. 1 herein is a reproduction of FIG. 1 of Yamamoto et. al. with the exception of the reference numbers; citations of reference numerals relate to FIG. 1 herein. Yamamoto, et.al. provide a compliant member 24 between integrated circuit package 12 and heat transfer element 14. Compliant member 24 is stated to improve the thermal contact between integrated circuit package 12 and heat transfer element 14; however, compliant member 24 increases the number of thermal interfaces, by adding thermal interfaces between integrated circuit element 12 and compliant member 24 and between compliant member 24 and heat transfer element 14.
Each thermal interface reduces the ability of the integrated circuit assembly to transfer heat generated by the integrated circuits. Conventional cooling systems have been modified over time to maintain an acceptable temperature of an integrated circuit generating up to approximately 30 watts. This upper limit on the integrated circuit power generation is due, at least in part, to the large thermal resistance of the structure provided between the integrated circuit and the coolant in the bellows. The thermal resistance of conventional devices is on the order of 0.6.degree. to 1.degree. C./watt. The thermal resistance of a structure is a ratio of the temperature differential between the surfaces of a structure which transfers heat from one surface to the other to the power transferred. Further discussion regarding thermal resistance may be found in Fujitsu Soientific Technical Journal, 23.4, page 243-254, Deoember 1987.
Several other problems are associated with the conventional integrated circuit assembly shown in FIG. 1. First, in order to improve thermal contact the surfaces of elements 12 and 14 must be extremely flat, which requires expensive manufacturing techniques. Second, bellows 16, which is usually plated with gold, is expensive both from a materials and a labor standpoint. Third, accessibility is hampered because compliant member 24 tends to adhere to the surfaces of elements 12 and 14, making it difficult to disassemble the integrated circuit assembly for testing and reworking during manufacturing.
A further problem with a conventional liquid cooled integrated circuit assembly is related to the pressure applied to printed circuit board 10 by bellows 16. Mechanical pressure is created by bellows 16 which forces the heat transfer element 14 attached to the bellows 16 into contact with integrated circuit 12. In addition, hydraulic pressure is operated by pumping coolant into cavity 18 in bellows 16. The combined pressure of a large number of bellows 16 contacting a large number of integrated circuits 12 mounted on a single printed circuit board 10 creates stresses in the printed circuit board 10. This stress leads to damage to the printed circuit board 10 and the electrical circuits therein, reducing the reliability of the printed circuit board 10 and the integrated circuits 12 mounted thereon.
In order to create mechanical pressure against the integrated circuits, conventional integrated circuit assemblies utilize a rigid header 26 mounted to the printed circuit board 10 by supports 28. The combined mechanical and hydraulic pressure created by all of the bellows 16 creates a large amount of stress on the printed circuit board 10.